This study will test the hypothesis that plasma arginine levels in critically ill pediatric patients are affected by their severity of illness by measuring plasma arginine levels concurrently with administration of the PRISM III (Pediatric Risk of Mortality) Score. By measuring the correlation between the arginine levels and PRISM scores at the beginning, in the middle, and at the end of critical illness we will test the hypothesis that arginine levels fall inversely with the severity of illness and rise as the patient improves.